1. Field of the Invention
The present invention relates to an image forming apparatus and a control method of the same.
2. Description of the Related Art
An image forming apparatus, such as a copying machine, a facsimile machine, and a printer, includes a photosensitive element as an image carrier that rotates in the sub-scanning direction in accordance with an image of an original or image data of the image. The photosensitive element is uniformly charged by a charging unit, and exposed by an exposing unit to form an electrostatic latent image. The electrostatic latent image is developed by a developing unit using a toner to form a toner image, and the toner image is transferred by a transfer unit onto a sheet as a recording medium. The toner image on the sheet is then fixed by a fixing unit including a fixing roller with a heater and a pressure roller by applying heat and pressure.
The fixing roller and the pressure roller are in contact with each other with a predetermined pressure in the fixing unit. At this state, a contact width (a nip width) between the rollers needs to be maintained at an appropriate value. For example, Japanese Patent Application Laid-Open No. 2004-109521 discloses a conventional technology for providing a fixing unit of which the nip width can be changed to attain an appropriate nip width depending on types or thicknesses of sheets.
However, even in such a fixing unit, if the fixing roller and the pressure roller are stopped while being in contact with each other at the same position for a long time, the contact surfaces of the rollers are deformed to a flat shape. As a result, the circularity of the rollers is degraded to an irregular shape (flat spot state), resulting in a noise or a nonuniform fixing of the toner when the fixing roller and the pressure roller rotate.